An optical communications system comprises a light source, such as a laser, a medium through which the light is transmitted, such as an optical fibre and a means of detecting the light, such as a photodetector. The component containing the light source is known as the transmitter and the component containing the detecting means is known as the receiver. The purpose of a communications system is to transfer information from one place to another. The output from the light source is modulated to encode this information and the modulation is detected by the receiver such that data is transmitted through the system.
The term “light” with reference to an optical communications system is used herein to refer to electromagnetic radiation from any part of the electromagnetic spectrum.
As the requirement to transfer data across an optical communication system increases, there are two techniques which are commonly used to increase the system capacity. The first technique involves increasing the rate at which the light is modulated which permits more data to be transmitted in a given time. The second technique involves using more than one wavelength of light to transmit more than one stream or channel of data concurrently. This second technique is known as wavelength division multiplexing and an optical communications system which uses this technique is called a wavelength division multiplexed system, or WDM system. The first and second techniques are often used in combination by both increasing the modulation rate and using more than one wavelength of light. In reality a stream of data is not transmitted on a single wavelength, but on a small range of wavelengths. The size of the wavelength range is dependent on the exact system implementation, but is typically 0.3 nm for a 10 Gbit/s system. Hereafter the term “wavelength” with reference to a channel of data is taken to mean the centre wavelength of the transmitted light, accepting that there will be a small spread of wavelengths around this value.
A schematic diagram of a simple WDM system is shown in FIG. 1. The system comprises a plurality of transmitters 102 and the output light from each transmitter is of a different wavelength. The output light from each transmitter is combined on to a single transmitting medium 104, such as an optical fibre, by means of an optical multiplexor 106. At the other end of the system, the individual wavelengths are separated by means of an optical demultiplexor 108 and detected by different detectors 110. The optical multiplexors and demultiplexors 106, 108 are examples of optical filters.
An example of a multiplexor or demultiplexor is an arrayed waveguide grating (AWG) or PHASAR based device, hereafter referred to as an AWG. Such devices are discussed in detail in ‘PHASAR-Based WDM-Devices: Principles, Design and Applications’ by M. K. Smit and Cor van Dam, published in the IEEE Journal of Selected Topics in Quantum Electronics, Vol. 2, No. 2, June 1996, and a basic description is included here.
A schematic layout of a simple 1×4 AWG is shown in FIG. 2. This device has 1 input port 202 and 4 output ports 204. The AWG additionally comprises 2 free propagating regions 206, 208, also known as star couplers, which are connected by a plurality of optical waveguides 210, each of which has a different optical path length. These waveguides 210 are hereafter referred to as the array of waveguides.
The operation of the AWG shown in FIG. 2 when used as a demultiplexor is as follows. A beam of light propagates down the input port 202 and when the beam of light enters the star coupler 206 it is no longer laterally confined and the beam diverges. At the other end of the star coupler 206, the beam is coupled into the array of waveguides 210, and is propagated along these waveguides to the second star coupler 208. The length of the waveguides within the array increases linearly across the array. This results in the focal point moving along the output plane of the second star coupler 212 as the wavelength changes. By placing the output guides 204 at the appropriate positions along this plane 212, a different wavelength or range of wavelengths is coupled to each output port.
The operation of such an AWG is reciprocal, such that the device shown in FIG. 2 could also be used as a multiplexor with guides 204 operating as 4 input ports and guide 202 operating as a single output port. The AWG operated in this manner would combine the 4 different wavelengths input one on each of the ports 204 onto the output port 202.
The term “reciprocal” with reference to the operation of an AWG is used herein to mean that the operation of an AWG is substantially reversible. This can be described with reference to FIG. 2, such that if the AWG was used as a demultiplexor and an input signal on port 202 contained 4 wavelengths, λ1, λ2, λ3, λ4, wavelength λ1 would be output by the first of the output ports 204, wavelength λ2 by the second etc. However, if the same AWG was used as a multiplexor, waveguides 204 become the input ports and waveguide 202 the output port, and if an input signal of wavelength λ1 was input on the first input port 204, λ2 on the second, λ3 on the third and λ4 on the fourth, all 4 wavelengths would be combined and output via the output port 202.
An AWG as described above has a pass band shape which is substantially Gaussian. FIG. 3 shows a typical Gaussian transmission profile for 1 of the outputs of an AWG as shown in FIG. 2. In some optical communication systems it is attractive to flatten the pass band of such filters on account of the insensitivity of their insertion loss to the frequency of the transmitted optical signal. FIG. 4 shows an example of a transmission profile for 1 of the outputs of a pass band flattened AWG.
A method of pass band flattening is detailed in U.S. Pat. No. 5,629,992 and FIG. 5 shows a schematic layout of a pass band flattened 1×4 AWG. The AWG in FIG. 5 has all the features of that shown in FIG. 2 and common features have been labelled with the same numbers; an input port 202; two star couplers 206, 208 connected by an array of waveguides 210, and 4 output ports 204. Additionally there is included a multimode element 502. This multimode element is a region which allows propagation of multiple modes of light and could comprise a waveguide structure or a region of free space. When applied to an AWG device, this multimode element can be implemented as a substantially cuboid section of waveguide, hereafter referred to as an MMI section. The MMI section works by being excited by a zero order mode injected into the centre of the input side from a narrower single mode guide. The narrow input field profile excites a mixture of the zero and second order modes in the MMI section, which then move in relative phase by pi radians along the MMI section on account of the different propagation constants of the two modes. The two modes have therefore inverted in relative phase at the output of the MMI section into the star coupler giving rise to a semi-flattened field distribution. The field profile emanating from the MMI section is substantially re-imaged in the output plane of the second star coupler 212. The position of the field profile depends on the wavelength as already discussed. The filter transmission response is given by the overlap integral of the field distribution at the output plane and the zero order mode field profile guided in the output port as a function of wavelength.
FIG. 6 shows an expanded view of the MMI section in FIG. 5. The light is travelling in the direction shown by the arrow 602 from the input guide 202, into the MMI section 502 and then into the star coupler 206. The progression of the field profile through the MMI section as described above is shown by the profiles 604.
Although the techniques described above using MMI sections produce optical filters with flattened pass bands, measurements of such AWGs show a problem with the pass band shape. If an input/output waveguide is not aligned with the centre of the star coupler the pass band has a significant slope. FIG. 7 shows an enlarged view of a section of FIG. 5. The figure shows the second star coupler 208 and output waveguides 702, 704, 706, 708. The centre line of the star coupler is marked 710 and it can be seen that none of the output guides lie on that line. All 4 outputs will therefore suffer slope on the pass band. The degree of slope scales with distance from the centre line measured along the plane of the output guides 212, such that the slope on the pass band of output port 702 will be more severe than that on output port 704. Simulated pass band shapes for output ports 1 and 40 of a 1×40 AWG with a channel spacing of 100 GHz fabricated in silica on silicon planar waveguide technology are shown in FIGS. 8 and 9.
The slope on the pass band is undesirable as it leads to a reduced filter bandwidth, which degrades the performance of the optical communications system.